Train rails are joined together via a seriously awesome process called exothermic welding, which basically involves lighting off a chemical called Thermite and sending molten iron into a sand mould. Here’s how the fiery process works.

On many older railways, train rails were joined via jointbars—basically big rectangular plates of iron bolted to two adjacent rails. Today, you see all sorts of methods that join two rails chemically to create a continuously welded rail—methods like flash-butt welding, gas-pressure welding, enclosed-arc welding and the one in the video below: thermite welding.

Thermite welding is used all around the world, partly because it doesn’t require an electric power source, so it can be done wherever, and partly because it doesn’t require much skill compared to other processes.

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For it to work, technicians must cut two adjacent rails precisely so there’s a particular gap, often an inch or so. Then they clamp and seal sand moulds to each rail section, ensuring first that the two rails are perfectly aligned.

Then, in an effort to prevent bubbles by removing any moisture in the void, and to make sure the rail ends melt, workers pre-heat the ends of the rails with gas torches.

Once that’s done, they affix a crucible filled with a particular thermite compound (iron oxide and aluminum, plus an alloying agent added to the mixture to match the composition of the rails) on top of the mould, and light the thermite off using a high-temperature magnesium sparkler.

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The thermite melts, yielding hot iron and alumina, which melt a plug in the bottom of the crucible, sending the iron into the void. The alumina slag flows in last since it has a lower density, so it overflows and is gathered in catch pans on the side of the rail.

After a few minutes, the metal hardens, the moulds are removed, and the welds are cleaned up with hammers, chisels and grinders.